T. congolense Lister strain 1/148 has been cloned and passaged through Glossina morsitans morsitans and re-cloned. The resulting sub-strain has undergone a relapsing infection in a mouse. Variant specific glycoproteins (VSG) have been isolated from the initial and relapsing infections and partially characterized by chemical and immunological means. Studies are proposed to examine the mechanisms by which change from one VSG to another occurs. These studies will be carried out both on the phenotypic and the genotypic level.